Top: relative abundances of N, N₂ and CO as a function of depth into a translucent cloud with χ = 1, T = 30 K and n_H = 10³   cm^-3. Bottom: photodissociation rates of N₂ (black) and CO (red) as functions of depth. Alternative photodissociation rate curves (, green) consider shielding by dust alone (dashed), dust + self-shielding (dotted), and dust + H + H₂ (solid). The conversion from N to N₂ occurs at A_V ≃ 1.5 mag, as shown by the vertical dashed line.